So in my experience, the earlier you submit your complete application, the better. They also won't review it until it is complete. Given you are taking the MCAT at the end of September, your AMCAS application won't be fully complete until late October at the earliest (if someone has taken the MCAT during this time, say how long it took). Once it is complete and submitted, Mercer will receive your primary. The past several times I've done this, the secondary comes usually after a week or two. COVID may change this. Given the time, you'll only be able to apply in the regular pool and not early decision or assurance.
This is the timeline for admissions for what is due:
medicine.mercer.edu